Innate immune responses to Streptococcus pyogenes

The Gram-positive bacteria Streptococcus pneumoniae and Streptococcus pyogenes are relevant pathogens responsible for mild and severe diseases in humans and a major cause of morbidity and mortality worldwide. The emergence and spread of pneumococcal clinical isolates resistant to antibiotics have become a major concern. The last thirty years have also witnessed a significant increase in the incidence of severe forms of S. pyogenes infections. The project I288-B09 funded by FWF aimed to give an insight into the molecular mechanisms of S. pneumoniae and S. pyogenes recognition by macrophages. The initial recognition of bacterial pathogens by the innate immune system is critical to launch innate and adaptive immune responses and, consequently, for the outcome of infection. An inappropriate recognition may result in insufficient immune responses, yet an over-activation of the immune system may be equally deleterious. Findings from my laboratory had indicated that yet unidentified molecules are involved in the recognition of S. pyogenes by macrophages. We had shown that S. pyogenes is recognized by a MyD88-dependent receptor other than any of the Toll-like (TLR) receptors typically used by bacteria. In this project, while my colleagues of the ERA-NET consortium (Austria, France, Finland) focused on the identification of streptococcal pattern-recognition receptors (PRRs) of macrophages, the specific aims of my laboratory were to identify the pathogen-associated molecular patterns (PAMPs) of S. pyogenes that are recognized by macrophages to trigger cytokine production. For this purpose, we used a genetic approach and engineered a library of S. pyogenes Himar1 transposon mutants that was screened in high- throughput infection assays of macrophages for increased and decreased levels of cytokine production. Mutants affected in the production of TNF-alpha were selected and the genes responsible were identified by gene walking. We selected 10 candidates for a detailed analysis of their effect on the host inflammatory signalling pathways. Together, the project allowed to establish the genetic tools, mutant libraries and screening methods that will enable to pursue the screens for PAMPs not only involved in TNF- alpha production but also in interferon production by macrophages as well as dendritic cells. We expect that future work stemmed from this project will lead to a detailed overview of the S. pyogenes PAMPs involved in the recognition by innate immune cells. Research in this direction may also lead to the identification of promising targets for the modulation of the immune response to this pathogen.